My (non-Thai) son has been a student there since P2. He's now P4. He loves it and I'm very happy with how he has adapted from an International School to a Bilingual English Program school - his Thai reading & writing skills have improved immeasurably.
Thailand doesn't have a 6 months rule, it just requires a passport that is valid for the duration of the visit. Unfortunately, a lot of airlines (and agencies) err on the side of caution and might not let you check in.
Mainly because it's fun, the girls (or boys) are sexy and often very attractive and it's cheaper than utilising the services of prostitutes in most people's home country. In fact, it's far removed from the prostitution you find in most parts of the world.
Not just Bangkok and Chiang Mai - pretty much all over Thailand and SE Asia, except in the deep south. Just something you have to put up with if you have to live here.
Not really. The minimum age for prostitution is 18 in most countries, including Thailand.
As @Liverpool Lou has pointed out, paying for sex in Thailand is not, in itself, illegal but if the provider is under 18, it most definitely is!